The palm of the hand is made up of bones called metacarpals, and a metacarpal connects each finger and thumb to the hand. Each finger and thumb is formed of bones called phalanges. The connection of the phalanges to the metacarpals is called a “knuckle” joint or metacarpophalangeal joint (MCP joint), and acts like a hinge when the fingers or thumb are bent.
In each finger, there are three phalanges that are separated by two joints called the interphalangeal joints (IP joints). The proximal IP joint (PIP joint) is the one closest to the MCP joint. The other joint closest to the end of the finger is the distal IP joint (DIP joint). The thumb just has one IP joint.
The joints are covered on the ends with articular cartilage. Damage to the joints may occur as a result of arthritis, a sprain or fracture, and wherein the damage either directly or indirectly affects the articular cartilage. Typically, the joint does not line up the same after the injury and causes unusual wear on the articular cartilage, eventually damaging the articular surface and causing pain and loss of mobility.
Typical methods of surgically treating a damaged joint include artificial joint replacement or fusion. Fusion (arthrodesis) is used to enable bones that make up a joint to grow together into one solid bone. Fusions are commonly used in the PIP or the DIP joints in the fingers because it is easier than replacing the joint and is an acceptable alternative to replacement in many cases.
Existing methods of fusion are inadequate, such as (a) K-wire fusion, (b) or inserting a screw through the tip of the finger and through the joint to be fused because the joint is typically fused straight (i.e., without a bend in it), which is not a natural position for the joint of a finger during normal use. Herbert and Acutrack screws and their variants have been used, but by using these, the joint (DIP joint) and end of the finger are fused in a straight position, which is unnatural since the joint is normally bent during use. DIP fusions with angles can be performed but the process is technically demanding, so it is rarely performed. Additionally, the ability to angle the joint is limited and the bone purchase is poor. An example of prior art screws is shown in FIG. 1.
Other techniques such as pin and tension band or cerclage wire do not adequately solve the problem of easily fusing a finger joint in a bent position.